White-necked raven
| White-necked raven | ||||||||||
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White-necked raven ( Corvus cryptoleucus ) |
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| Corvus cryptoleucus | ||||||||||
| Couch , 1854 |
The Chihuahuan Raven ( Corvus cryptoleucus ) is a songbird art from the family of corvids (Corvidae). With a body length of 46–53 cm, it belongs to the medium-sized species of ravens and crows ( Corvus ) and inhabits southern North America from central Mexico to the southwestern Great Plains . Its habitat is formed by arid prairies and semi-deserts with few trees. White-necked ravens feed on a wide range of insects , small vertebrates , carrion, and grain . The species is very gregarious and occurs in large schools in winter. The birds' breeding season is in spring and early summer, and they build their nests in free-standing trees and human structures.
The Chihuahuan Raven is comparatively sparse researched and was only in 1845 by Darius N. Couch firstdescribed . It is closely related to the West American population of the common raven ( Corvus corax ) and was probably separated from it during the Plio or Pleistocene . The total population of the species is not recorded. Since the population development appears stable, the white-necked raven is managed by BirdLife International under least concern (“not endangered”).
features
Build and color
White-necked ravens reach body lengths of 46–53 cm. This makes them one of the larger ravens and crows , but they are smaller than the largest ravens. Compared to similarly large American crows ( C. brachyrhynchos ), they are characterized by stronger, curved beaks and wedge-shaped tails. Males of the species are on average larger and heavier than females, but the sexes clearly overlap in all dimensions. Males weigh between 442 and 667 g. Its hand wing is 332–381 mm long when extended, its tail measures 181–229 mm. Their tarsi are 55–65 mm long, the beaks of male white-necked ravens reach a length of 53–60 mm. Females reach a weight of 378–607 g and a wing length is between 324 and 362 mm. The female tail is 179-222 mm long, the barrel of female white-necked ravens measures 56.4-61.9 mm when fully grown. Their beak is 49–56 mm long.
The plumage of the species is almost completely black, but its individual areas show subtle differences in the intensity and shade of coloration. The visible plumage is glossy black and covered on the upper side of the body by a bluish or purple metallic sheen, while the outer flags of the hand wings and the underside of the body show only a faint greenish to bluish shimmer. The neck plumage is tinged brown on the sides. Its feathers are snow-white at the base, which is often only apparent when the plumage is disheveled. In some individuals the top breast feathers also have a light base. The throat feathers are elongated and lanceolate, which makes the throat feather appear disheveled and clearly visible. The beak of the species is shorter than that of the common raven ( C. corax ) and stronger and more curved than that of the American crow ( C. brachyrhynchos ). The long nasal bristles usually cover more than half of the beak. Like the legs, it is black or dark slate gray in adults. Young animals are characterized by pink areas in the throat and on the edges of the beak. Their plumage is comparatively dull and loose. The iris of white-necked ravens is yellow-brown to black-brown, age-related differences in their coloration are not known.
Flight image and locomotion
White-necked ravens usually stride on the ground. They jump over smaller obstacles. The flight of the species is lively and is characterized by moderately deep, measured wing beats, as they are also typical for other Corvus species. In cross-country flights, the animals usually move at low heights of around 20 m. Occasionally, however, they also rise to heights of over 100 m in order to circle there. To gain altitude quickly, Weißhals Ravens like to use whirlwinds and warm updrafts . Compared to the American crow, the tail of the white-necked raven is longer and more wedge-shaped in flight; their wings are also more pointed and deeper fingered than those of their smaller relatives. Common ravens, on the other hand, are very similar to white-necked ravens in flight and are outwardly characterized by an even longer and more wedge-shaped tail.
Vocalizations
The vocalizations of the species are only sparsely researched. Like other ravens and crows, the white-necked raven has a rather rough, croaking voice and an extensive vocabulary. The typical vocal feel for the genus kaa is used by birds in many different contexts. The intonation varies depending on the context: As a threatening sound, the call is lower than an alarm signal, for example. Depending on the situation, it is released individually or in series. Long, fast series usually indicate danger. Conflicts between individuals are sometimes accompanied by growling noises, courtship rituals by softer ag-ag calls. The high, chirping begging sounds of the young animals are used by adult birds as a signal for submission.
distribution
The distribution area of the Weißhalsraben includes the arid and semi-arid regions of central and southern North America . The distribution history of the species is only incompletely documented. In the late 19th century, the species was distributed in the north to the southern borders of Wyoming and Nebraska . From there, the distribution limit ran southeast through western Kansas and Oklahoma and further south through Texas , where the species area was roughly bounded by the 150 mm isohyete or the grain cultivation based on it. From there it spread along the Gulf Coast to Mexico , where the species' historical occurrence is not known. In the west, the Artareal encompassed the outer south of Arizona , large parts of New Mexico and western Colorado . Until the 1940s, the northern limit of distribution shifted southwards to the Texas border. In Colorado only a relic population remained, New Mexico was only settled in the south and east. The eastern limit of distribution in Texas receded behind San Antonio and only reached the Gulf Coast in the extreme south.
Today the white-necked raven lives in Mexico southwards to about Jalisco . From there, its distribution area runs to the northwest, where it is flanked by the Sierra Madre Occidental . The humid east side of the mountain is not colonized by the species. North of the mountains, the Artareal extends into northern and central Sonora . From there, its border runs through the outer southwest of Arizona and includes southern and eastern New Mexico. It is questionable whether the species occurs further northwest. A small relic population in the central border region of Colorado and New Mexico west of the Rio Grande Loop may indicate this. The white-necked raven populates the southeast of Colorado beyond the Arkansas River , where the species reaches the northernmost point of its distribution. In neighboring Kansas, the river forms the northern limit of its distribution for a short stretch before it turns southwest at Dodge City and crosses the outermost Oklahoma Panhandle . From there it runs to Texas, spares the far northeast of its panhandle and includes a narrow strip of Oklahoma in the east. From here the border runs in a slight curve southwards to the Gulf coast, which is followed by the distribution area to the center of Tamaulipas , the south of which is predominantly no longer one of the breeding areas. From the Gulf coast, the breeding line runs in a north-westerly direction, spares a small part of Nuevo León and then reaches its southernmost point in a wide arc with Guanajuato . In winter, due to the changed seasonal food supply, migratory movements take place. The species does not completely give up its breeding grounds, but parts of the local populations move to areas with increased occurrence of grain and other food. In the south of the distribution area, the winter quarters extend beyond the breeding areas: to the west of the Sierra Madre Occidental they encompass almost all of Sonora and Sinaloa . In central Mexico they extend westward from the southwestern Gulf coast to Jalisco, where they reach their most southwestern point and are in turn bounded by the Sierra Madre Occidental. The train movements start in early autumn and last until December. Towards the end of February the animals begin to migrate back to the breeding areas.
It is unclear what caused the species to retreat from the north of its range, which began at the end of the 19th century. Possibly the wide spread of the species in the north represents a reaction to the massive shooting of the bison ( bison bison ) by humans. As a result, large numbers of carcasses were often left on the prairie , which could have provided a rich source of food for the white-necked raven . The extinction of the species may have been delayed by the advent of extensive grain fields in Kansas and Colorado. Conversely, the species probably already spread before the end of the 19th century, after the erection of telegraph poles and other structures opened up treeless regions as breeding areas. One can only speculate about the prehistoric distribution of the white-necked raven. The closest relatives of the species, the ravens of California and Nevadas, suggest an origin of the species in the ice-free California of the Plio-Pleistocene . This hypothesis is supported by bone finds in the La Brea Tar Pits , which are similar in size and shape to those of the white-necked raven.
habitat
The white-necked raven is a typical prairie and desert inhabitant. It prefers dry, open grasslands, as occurs extensively in the flatlands of the USA and Mexico. The vegetation there is characterized by grasses of the genera Aristida , Bouteloua , Sporobolus , Muhlenbergia and Hilaria . Scattered mesquites ( Prosopis spp.), Palm lilies ( Yucca spp.) Or acacias ( Acacia spp.) Are often the only trees and shrubs in these landscapes. Particularly high settlement densities are reached in communities of Prosopis glandulosa , Bouteloua eriopoda and creosote bush ( Larrea tridentata ). The species can also be found in more desert-like landscapes. These types of vegetation, which are particularly common in the southern species area, are characterized by creosote bushes, the oak species Quercus havardii and various mesquite species . In the hill country, white-necked ravens can also be found in gappy Pinyon-Juniper-Woodland . In higher altitudes and denser forms of vegetation, they are replaced by the common raven ( C. corax ), which usually lives in the more humid regions where both species occur sympatricly . In the north of the range, the white-necked raven is also found in treeless prairies , where electricity pylons or wind turbines act as breeding grounds. In winter the species can also be found away from potential breeding grounds. In particular, extensive grain fields or rubbish dumps then become attractive as feeding habitats. Although the species of humans generally avoids densely populated areas, they show a high affinity for anthropogenic landscapes with lower population densities. The reasons for this include improved breeding sites, a high number of grazing cattle and the availability of grain seeds in winter.
Way of life
nutrition
As omnivores, white-necked ravens use a wide range of food sources. Depending on availability and needs, the seasonal composition of your diet varies greatly. Grain is of great importance for most of the year, but falls significantly behind insects and wild fruits during the summer breeding season . After insects, mammalian asas are the most important source of protein for animals and, as such, are particularly important in the autumn and winter months. In early summer, the proportion of locusts (Orthoptera spp.) In the diet increases in some regions to up to 60% of the total volume. The breeding season of the white-necked raven coincides with the main flight time of most orthoptera species, which also serve as main food for the nestlings. In late summer and early autumn, wild fruits in the diet increase sharply, while invertebrates fall behind. Later in the year, plant-based food becomes even more important. In the south of the USA, commercially grown sorghum species make up a large part of the diet. It is not until around March that animal food, which then occurs again in greater numbers, takes up a larger proportion than vegetable food. Spread over the year in a Texas study area, the following picture emerged: Sorghum millets consumed 26.66% of the food volume, followed by mammals (mainly carrion) with 13.63%. The third largest volume share was made up of locusts with 12.56%, beetles (Coleoptera) made up 9.55%, while field crops (excluding grain) made up 7.27% and wild fruits made up 7.00% of the annual food volume. Butterflies and their caterpillars accounted for a further 6.64% . Animal and vegetable food each made up around 50% of the total volume.
The birds look for their food mainly in the fields , but also from sitting areas or from flight. Prey animals are usually caught on the ground, rather rarely (in the case of grasshoppers, for example) from the air. In dying lambs and calves, the birds first eat the soft, hairless parts such as eyes, navel or anus, because they find it difficult to separate the skin of larger animals. Generally they approach dying or dead animals (such as roadkill ) with great caution and only dare to eat from them after a while. White-necked ravens look for insect larvae under animal manure and stones and often try different ways to open possible hiding places for insects. Conversely, they hide excess food in self-dug holes in the ground and under leaves or bark in order to retrieve it at a later point in time. White-necked ravens recycle human waste less often than American crows or common ravens, for example, because they tend to avoid more densely populated areas. Even so, they are often found in large numbers near camping sites or landfills, where they look for litter and discarded food.
Social behavior
Unlike its closest relatives, the white-necked raven is a very sociable bird. While most of the larger Corvus species live in pairs all year round as adults and defend larger territories, white-necked ravens can often be found in swarms of several hundred to ten thousand individuals in winter. These swarms wander together and can often be seen at water and food sources. Throughout the year, young birds and other non-breeders in particular use shared roosts, where 50 to 200 animals can be found. Breeding pairs separate from the swarms during the breeding season, but come back to them after the young have fled. In late summer, the sleeping communities often grow to over 500 birds. White-necked ravens are occasionally kept with American crows at roosts. The individual distance is usually less than 1 m in field swarms and in waiting areas. Within swarms, a loose hierarchy usually develops, in which dominant animals can drive away via subordinate to feeding places or sitting areas. Conflicts take place primarily in the form of chases, but rarely turn into direct physical aggression.
Reproduction
Data on the breeding biology of white-necked ravens are mainly available from the southern United States. In New Mexico and Texas, the first breeding pairs form from March and then appear more and more frequently in April and May. Mating is usually found in smaller, loose flocks. The courtship behavior consists of a ritual in which birds interested in each other initially line up parallel to each other at a distance of about 1 m and look in the same direction. From time to time one of the two birds walks in an arc towards the other, whereupon both begin to beak together and preen each other's plumage. After about a minute, both birds return to their starting position. Acrobatic courtship flights probably also play a role in reproduction.
Nest building begins in the southern USA in April. Trees, bushes, telephone poles, wind turbines or winding towers are chosen as nesting sites. In trees, the nest is usually placed in forks of branches; Due to the low growth of the trees in the region, they are often only 2–5 m high. On the other hand, the animals nest higher on anthropogenic structures; Depending on the design, the average nesting height is between 7 and 20 m. The nest consists of a 31–62 cm wide and 30–36 cm deep outer shell made of relatively long, bulky twigs (mostly from mesquites ) or pieces of wire that are loosely woven together. A 15–21 × 13–15 cm hollow is pressed into it and lined with finer materials such as hair, feathers, paper or grass. The actual construction work is mainly done by the female, while the male concentrates on collecting material. The female lays her greenish, often darkly speckled or dashed eggs from April to June. The clutch size is between one and eight eggs, the mean value fluctuates between 4.8 and 5.4 eggs depending on the year and region. The eggs are incubated for 18–22 days before the young hatch. The nestlings begin to climb branches after 30 days, but only fly out after about 35 days, and usually only after 37-40 days. At first it is very difficult for them to take off from the ground, which is why they climb onto bushes after landing and use them as a take-off point. Two days after leaving, they are able to cover distances of around one kilometer in flight. The reproductive rate in a Texas study was between 1.9 and 2.6 flown out pups per female. The ratio of fled-out young to laid eggs in the US has historically been between 50 and 89%. These relatively large fluctuations are probably related to the availability of locusts in the respective years, which are the main food of the nestlings during their flight time.
Diseases and Causes of Mortality
Due to their size, white-necked ravens have relatively few enemies. The death of adult animals by predators is only documented in one case in which the remains of an individual were found in a peregrine falcon nest . Nestlings, on the other hand, are more likely to fall victim to typical nest robbers. Coyotes ( Canis latrans ) occasionally shoo half-fledged young birds out of deep nests and then catch them on the ground when they land exhausted. In addition to predators, strong winds and starvation are common causes of death among nestlings. The main causes of death for adult white-necked ravens are probably being shot down by humans (which can also result in the death of nestlings), electric shocks from high-voltage lines or collisions with cars, as have also been documented for other scavengers. However, there are hardly any quantitative data for the species: of over 1700 birds ringed in 2012, only 10 were spotted again by spring 2013. This makes it difficult to make general and representative statements about the total population and common causes of death. The same applies to life expectancy. Reliable data are only available for maximum values: the oldest known white-necked raven was ringed in 1980 when it was already at least one year old, and was spotted again in 2001. The second oldest recorded individual lived to be at least twelve years old. Regardless of such long life spans, many young animals probably die in their first year of life.
Among the ectoparasites of the species, the featherlings Brueelia afzali and Philopterus ocellatus osbornis stand out, but the white-necked raven serves as a host for many other jaw lice . The most common internal parasites are the nematodes Acauria Anthurus and tricuspis Diplotriaena . Every wild white-necked raven is usually infested with parasites, but this does not necessarily result in health problems. The high level of parasitism is probably due to the fact that the species is extremely sociable and birds sleep and eat close to one another.
Systematics and taxonomy
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| External classification of the white-necked raven according to Baker & Omland (2006). The species stands within the common raven complex and makes it paraphyletic . |
The white-necked raven was first described in 1853 by Darius N. Couch , who was then on an ornithological expedition in northern Mexico. Couch described a female from Tamaulipas as a holotype and gave the species the epithet cryptoleucus (from κρύπτον / krýpton , "hidden", and λευκός / leukos , "white") with a view of the hidden white neck feathers .
An early study based on mtDNA sequences by Kevin Omland and colleagues in 2000 came to the conclusion that the white-necked raven is closely related to the common ravens of the North American west coast ( C. corax sinuatus ), while the remaining ravens of the Holarctic formed a separate clade. Chris Feldman and Kevin Omland were able to reproduce this result again in 2005. This latter study also grouped the white-necked raven and the Californian raven as well as the Holarctic raven and the tortoise- shell raven ( C. albus ) together. Further investigations by the authors underlined these results. It is unclear what taxonomic consequences these results have: From a phylogenetic point of view , the white-necked raven should either be added to the common raven or the West American common raven raised to the rank of species. Since the West American ravens evidently reproduce with Holarctic ravens, while this cannot be observed in white-necked ravens, the biological species concept speaks in favor of maintaining the taxonomic status quo . The white-necked raven and the common raven probably separated from each other during the Pliocene (around 2.6-0.01 mya ) when they were isolated by repeated glaciations of the Rocky Mountains and subsequently developed different habitat preferences.
Usually no subspecies are distinguished within the white-necked raven, it is considered to be monotypical . The subspecies C. c. reai , which Allan Phillips established in 1986 for larger animals from the west of the distribution area, is apparently based on inaccurate measurements, which is why it is currently not considered valid.
status
No data is available on the total population of the white necked raven. Only the population development in the southern USA can be estimated using the Breeding Bird Surveys over the past 40 years. According to this, the breeding population within the observation areas has remained stable. Only in the Tamaulipan Brushlands on the lower reaches of the Rio Grande did the population decrease by 2.7% during the study period. Between the years - depending on the food supply - there can obviously be strong regional population fluctuations, as Mexican studies show. Possible dangers for the species are largely unexplored, but electric shocks from wire in the nesting material, the use of herbicides (especially of tebuthiuron against Quercus havardii ) and active human persecution apply . While targeted killing by shooting down or poisoning resulted in high losses, especially in the 20th century. In 1934, over 10,000 white-necked ravens were caught using four traps. In the meantime, however, the persecution has apparently decreased, while the danger of electric shock has moved into the focus of bird conservationists. It is estimated that the white-necked raven alone will die from several hundred to a thousand each year. As a countermeasure, specially adapted electricity pylons are used in some places in the USA, which reduce the risk of short circuits through nesting material.
Protection programs related to the white-necked raven do not currently exist in either Mexico or the USA; BirdLife International lists the species as harmless. However, the decline of the species in the north of its range is being watched with concern because it may indicate a decline in the short-grass prairie. The targeted maintenance of decommissioned telephone poles is being discussed among ornithologists as a conservation measure. In order to limit the damage caused by the species to grain fields, the shooting of birds, alarm firing systems and the extensive destruction of their nests were discussed in the 20th century, but only rarely carried out consistently. More recent proposals tend to be more in the direction of deterrent control measures and the protection of crops with textiles.
swell
literature
- Shaler E. Aldous: The White-necked Raven in Relation to Agriculture . In: Research Reports . tape 5 . US Department of the Interior, Fish and Wildlife Service , Washington, DC 1942 ( full text ).
- Jason M. Baker, Kevin E. Omland: Canary Island Ravens Corvus corax tingitanus have distinct mtDNA . In: Ibis . tape 148 (1) , 2006, pp. 174-178 , doi : 10.1111 / j.1474-919X.2006.00493.x .
- Julian J. Baumel: Individual Variation in the White-Necked Raven . In: The Condor . tape 55 (1) , 1953, pp. 26-32 , doi : 10.2307 / 1364920 .
- Darius N. Couch: Descriptions of New Birds of Northern Mexico . In: Proceedings of the Academy of Natural Sciences of Philadelphia . tape 7 , 1854, pp. 66-67 ( full text ).
- James F. Dwyer, James C. Bednarz, Ralph J. Raitt: Chihuahuan Raven (Corvus cryptoleucus) . In: A. Poole, F. Gill (Eds.): The Birds of North America Online . 2013, doi : 10.2173 / bna.606 .
- Chris R. Feldman, Kevin E. Omland: Phylogenetics of the Common Raven complex (Corvus: Corvidae) and the utility of ND4, COI and intron 7 of the beta-fibrinogen gene in avian molecular systematics . In: Zoologica Scripta . tape 34 (2) , 2005, pp. 145-156 , doi : 10.1111 / j.1463-6409.2005.00182.x .
- Elisabeth Haring, Barbara Däubl, Wilhelm Pinsker , Alexey Kryukov, Anita Gamauf: Genetic divergences and intraspecific variation in corvids of the genus Corvus (Aves: Passeriformes: Corvidae) - a first survey based on museum specimens . In: Journal of Zoological Systematics and Evolutionary Research . tape 50 (3) , 2012, p. 230-246 , doi : 10.1111 / j.1439-0469.2012.00664.x .
- Knud A. Jønsson, Pierre-Henri Fabre, Martin Irestedt: Brains, tools, innovation and biogeography in crows and ravens . In: BMC Evolutionary Biology . tape 12 (1) , 2012, p. 72 , doi : 10.1186 / 1471-2148-12-72 .
- Kevin E. Omland, CL Tarr, WI Boarman, John M. Marzluff, RC Fleischer: Cryptic genetic variation and paraphyly in ravens . In: Proceedings of the Royal Society B: Biological Sciences . tape 267 (1461) , 2000, pp. 2475-2482 , doi : 10.1098 / rspb.2000.1308 .
- Kevin E. Omland, Jason M. Baker, Jeffrey L. Peters: Genetic signatures of intermediate divergence: population history of Old and New World Holarctic Ravens (Corvus corax) . In: Molecular Ecology . tape 15 (3) , 2006, pp. 795-808 , doi : 10.1111 / j.1365-294X.2005.02827.x .
- Gary S. Pfaffenberger, Weldon F. Butler, DS Butler: New host record and notes on Mallophaga from the White-necked Raven (Corvus cryptoleucus Couch) . In: Journal of Wildlife Diseases . tape 16 , 1980, pp. 545-547 .
- Gary S. Pfaffenberger, Weldon F. Butler: Helminths Recovered from the White-necked Raven (Corvus cryptoleucus Couch) in eastern New Mexico . In: Journal of Wildlife Diseases . tape 17 , 1981, p. 563-566 .
Web links
- S. Butchart, J. Ekstrom: Chihuahuan Raven Corvus cryptoleucus . BirdLife International , www.birdlife.org 2013.
- Corvus cryptoleucus inthe IUCN 2013 Red List of Threatened Species . Listed by: BirdLife International, 2012. Retrieved September 7, 2013.
Individual evidence
- ↑ a b Couch 1854, p. 66.
- ↑ a b Baumel 1963, p. 28.
- ↑ a b c d e f g h i j k l m n o p q Dwyer et al. 2013. Retrieved May 3, 2013.
- ↑ Aldous 1942, pp. 4-6.
- ↑ Aldous 1942, pp. 19-20.
- ↑ Pfaffenberger et al. 1980, p. 546.
- ↑ Pfaffenberger & Butler 1981, p. 564.
- ↑ a b Baker & Omland 2006, p. 175.
- ↑ Omland et al. 2000, p. 2477.
- ↑ Feldman & Omland 2005, p. 151.
- ↑ Omland et al. 2006, p. 799.
- ↑ Omland et al. 2000, p. 2481.
- ↑ Jønsson et al. 2012, p. 8.
- ^ Butchart & Ekstrom 2013. Accessed May 4, 2013.